1. Field of the Invention
The present invention relates to a power converter using a field effect transistor and a diode.
2. Description of the Related Art
As a power converter using a wide band-gap semiconductor device in the related art, there is a voltage-sourced power converter. This power converter has two semiconductor switches each of which is formed of an FET (Field Effect Transistor) using a wide band-gap semiconductor and a free wheel diode using a wide band-gap semiconductor and connected inversely in parallel (connected in anti-parallel) to the FET. These two semiconductor switches are connected to a capacitor functioning as a voltage source. The power converter converts power by complementary switching actions of the FETs in the two semiconductor switches.
An FET and a diode both using a wide band-gap semiconductor are operable at high temperatures and have a small switching loss relative to an IGBT and a diode both using a silicon semiconductor. Accordingly, by replacing the silicon semiconductor with the wide band-gap semiconductor, it becomes possible to reduce an area of the semiconductor and to make a cooler cooling the semiconductor simpler. Also, because passive component parts, such as a capacitor and a reactor, can be miniaturized owing to high-frequency switching, it becomes possible to downsize the power converter.
When an FET is used as the semiconductor switching device, it is possible to use an internal parasitic diode (body diode) of the FET instead of using the free wheel diode. Nevertheless, the free wheel diode is used because a characteristic of switching actions can be improved by using a Schottky barrier diode capable of operating at a high speed as the free wheel diode.
Also, with an FET using an SiC semiconductor, which is a wide band-gap semiconductor, when the body diode is energized, it is anticipated that a bipolar action by the body diode promotes crystalline degradation in the SiC semiconductor. To avoid such an inconvenience, a free wheel diode having an ON voltage lower than an energization starting voltage of the body diode is used as described, for example, in Patent Document 1.
Patent Document 1: JP-A-2007-305836
The power converter in the related art, however, uses an FET that can be energized in two ways (directions from drain to source and from source to drain). Hence, while the FET is in an ON state, a current hardly flows through the free wheel diode and the current flows through the FET having a lower ON voltage than the free wheel diode.
Accordingly, during complementary switching actions of the FETs in the two series-connected semiconductor switches, a current flows through the FETs in periods other than a dead time. The FETs therefore generate heat and temperatures of the FETs rise in comparison with the free wheel diodes. Hence, there arises a problem that the temperatures of the FETs limit maximum output power of the power converter.